Hive Mind

You know how your brain is made of a bajillion on/off chemical switches? What if you had a meta-brain that was made of a bajillion brains? Hive minds make for good villains, since you can have from a dozen to a city full of faceless minions that never fail morale checks. Some Hive minds have a central hub 'queen', or multiple 'queens', while others have delegated thinking-drones with their own personalities.

Ant colonies are usually the textbook example of a hive-mind, but ants do not have any dedicated thinking units, nor any command structure; their queens are just immobile baby factories. Nothing does the thinking, but they act with predictable purpose and behaviour. Really creepy when you think about it too much.

Warhammer 40,000

The "Hive Mind" is the ultimate, supreme grand master awesome badass overlord of all tyranids. It blots out all psychic signals including the Astronomican, astropath communications, and even the warp. The latter is very upsetting to anyone that isn't a Tyranid (And probably Necrons. But the majority of them don't care either way).

The Hive Mind is made of literally every Tyranid creature. The hive fleets are merely appendages of the Hive Mind . This means it is very, very, very big; fucking huge, actually. Its capacity for violence and reach is far beyond that of Khorne's, renders Tzeentch's plans impotent (individual tyranids have no minds to outsmart), the immortality offered as part of a greater whole is beyond the shambling eternity of Nurgle, and no temptation nor depravity of Slaanesh's can sway a tyranid away from the Hive Mind any more than you or I could cajole a liver cell into acting like a kidney. The Emperor himself cannot penetrate the will of an entire spacefaring species acting as one. There are only two recorded cases of the Hive Mind being in any way foiled: Varro Tigurius (in a story written by Matt Ward, natch), and one case when a besieged craftworld managed to get a distress call out past the Hive Mind to the Eldar pirate prince Yriel (this fleshing out of lore was also written by Matt Ward; but here it was a concentrated effort by an entire planet of highly psychic aliens, and more than one died from the strain).

When facing a Tyranid in battle, it is not the individual before you that you fight, but the Hive Mind. While the Hive Mind does make mistakes and can be outsmarted, you can fool it with the same trick only once; losses are anticipated, and are useful either to cull the genepool, or as fuel expended to ensure victory. The Tyranid Codex reminds us that even defeats anywhere may serve to ensure a victory somewhere else -- pray that you are at "anywhere" and not "somewhere else."

If you want to know its full brilliance: it once outwitted the Ultramarines. There you go.

Fun Fact: The Hive Mind from DoW:retribution, Sindel from MK9 and Ivy from Soul Calibur all share the same voice actress.